1. Field of the Invention
The present invention relates, in general, to a light modulator having a variable blaze diffraction grating and, more particularly, to a light modulator having a variable blaze diffraction grating, in which a diffraction member rotates due to piezoelectric force so as to incline a reflective surface.
2. Description of the Prior Art
Generally, optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a large amount of data in real-time, and studies have been conducted on the design and production of a binary phase only filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
Of them, the spatial light modulator is applied to optical memory, optical display, printer, optical interconnection, and hologram fields, and studies have been conducted to develop displays employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspended on an upper part of a substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on an oxide spacer layer 12 using a nitride frame 20.
In order to modulate light having a single wavelength of λo, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λo/4.
Limited by the vertical distance (d) between the reflective surface 22 of each ribbon 18 and the reflective surface of the substrate 16, the grating amplitude of the modulator 10 is controlled by applying voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 of a lower side of the substrate 16 acting as a second electrode). In its undeformed state, with no voltage applied, the grating amplitude is λo/2 and the total round-trip path difference between light beams reflected from the ribbon and substrate is one wavelength λo, and thus; the phase of the reflected light is reinforced.
Accordingly, in its undeformed state, the modulator 10 acts as a plane mirror when it reflects light. In FIG. 2, reference numeral 20 denotes incident light and reflected light in its undeformed state.
When a proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to be moved downward toward a surface of the substrate 16. At this time, the grating amplitude is changed to λo/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 resulting from the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in a +/− diffractive mode (D+1, D−1) in a deformed state.
However, the Bloom's light modulator adopts an electrostatic method to control the position of the micromirror, which has disadvantages in that the operating voltage is relatively high (usually, 20 V or so) and the correlation between the applied voltage and the displacement is not linear, resulting in unreliable light control.
Meanwhile, Silicon Light Machines Inc. has suggested a blaze light valve device, in which blaze diffraction is conducted to control the intensity of light, as disclosed in Korean Patent Laid-Open Publication No. 2004-32908.
FIG. 4 shows a perspective view of a blaze grating light valve according to conventional technology. The blaze grating light valve 120 comprises a substrate 122, elongate members 124, first posts 126 (only one post is shown), and second posts 128 (only one post is shown).
The substrate 122 includes a first conductor 130. It is preferable that each of the elongate members 124 include reflective first and second surfaces 132, 134. The first and second surfaces 132, 134 form a blaze profile 136 for the elongate member 124. One of the first posts 126 and one of the second posts 128 function to connect each elongate member 124 to the substrate 122. Furthermore, the elongate member 124 is connected to the substrate 122 at first and second ends thereof (not shown).
FIG. 5 is a perspective view of one of the elongate members 124 and a portion of the substrate 122. Each elongate member 124 includes the reflective first and second surfaces 132, 134. The first and second surfaces 132, 134 form the blaze profile 136.
The elongate member 124 is connected through the first and second posts 126, 128 to the substrate at the first and second ends thereof (not shown). Preferably, the elongate members 124, the first posts 126, and the second posts 128 are made of an elastic material. It is preferable that the elastic material include silicon nitride.
Preferably, the first and second surfaces 132, 134 each include a reflector. It is preferable that the reflector include an aluminum layer. Alternatively, the reflector is made of another metal. Selectively, the reflector is a multilayer dielectric reflector. The substrate 122 includes the first conductor 130. Preferably, the substrate 122 includes silicone, and a first conductive layer is doped polysilicone. If a visible spectrum is used, the portion of the elongate member 124 between the first post 126 and the second post 128 has a length of about 200 μm and a width of about 4.25 μm.
FIG. 6 illustrates a second blaze grating light valve according to conventional technology. In the second blaze grating light valve 120B, a second elongate member 124C is used instead of the elongate member 124 of the blaze grating light valve 120. In the second elongate member 124C, a step profile 150 of the elongate member 124 is substituted by a flat surface 226 inclined at a blaze angle (γ).
Meanwhile, U.S. Pat. No. 5,311,360 discloses a conventional blaze diffraction grating, which diffracts light by inclining a reflective surface using electrostatic force, as described in an example (FIG. 7) thereof. However, the conventional blaze diffraction grating (disclosed in the patent application of Silicon Light Machines Inc. as well as U.S. Pat. No. 5,311,360) is problematic in that the generation force per unit volume is insufficient because of the use of electrostatic force, thus rotation displacement efficiency per unit input is poor.